Observation

The Chief Examiner reported that majority of the candidates could not recall the relationship between the various d.c. current flows in a bipolar junction transistor.The Chief Examiner reported that responses to this question were generally poor despite majority of it being ‘recall’.

The expected responses were:

1. 1. Modulation in transmitters refers to the process of changing some characteristics (amplitude, frequency and phase) of a high frequency signals by an information-bearing baseband signal. It involves the superimposition of information on a carrier by either varying the amplitude, frequency or phase of the carrier.
   
   
   2. Mixing is the process of multiplying/combining two sinusoidal signals at different frequencies, to produce two other signals, whose frequencies are one, the sum and the other, the difference, of the frequencies of the signals that served as input to the mixer.


2. 1. Video - Amplitude modulation
   
   
   2. Sound - Frequency modulation


3. 1. Horizontal frequency – 15625Hz
   
   
   2. Vertical frequency – 50 Hz